Apparatus and method for adjusting the spectral response of an optical waveguide grating

ABSTRACT

Apparatuses and methods for adjusting the spectral response of an optical waveguide grating include a longitudinal optical waveguide grating attached to a support member having a centroidal axis. The support member is configured for longitudinal bending and rotating about its centroidal axis. The spectral response of the optical waveguide grating is adjusted by bending and rotating the support member about its centroidal axis.

FIELD

This disclosure generally relates to optical waveguide gratings. Moreparticularly, this disclosure relates to an apparatus and method foradjusting the spectral response of an optical waveguide grating.

BACKGROUND

Optical fibers are thin strands of glass capable of transmittinginformation containing optical signals over long distances with low lossof signal strength. In essence, an optical fiber is a small diameterwaveguide consisting of a fiber core having a first index of refractionsurrounded by a fiber cladding layer having a second lower index ofrefraction. So long as the refractive index of the core is sufficientlyhigher than the refractive index of the cladding, a light beampropagating along the core exhibits total internal reflection and isguided along the length of the core. Typical optical fibers are made ofhigh purity silica, with various concentrations of dopants added tocontrol the index of refraction of the core and the cladding.

Many optical materials exhibit different responses to optical waves ofdifferent wavelengths. One such phenomenon is chromatic dispersion (orsimply “dispersion”) in which the speed of light through an opticalmedium is dependent upon the wavelength of the optical wave passingthrough the medium. Since the index of refraction for a material is theratio of the speed of light in a vacuum (a constant) to the speed oflight in the material, it therefore follows that the index of refractionalso typically varies as a function of wavelength in these materials.

Optical fibers are frequently used in data networks (such astelecommunication networks) requiring high data transmission rates. Inmany networks, data is also transmitted over very long distances. As thedistances over which data is transmitted increase, or the rates at whichdata is transmitted increase, chromatic dispersion presents obstacles toachieving error free performance. Specifically, in long distancetransmission of optical signals such as from a laser, the laserbandwidth and the modulation used to encode data onto the laser beamresults in a range of wavelengths being used to transmit theinformation. The combination of this range of wavelengths and thechromatic dispersion of the fiber accumulated over a distance results inpulse broadening or spreading. For example, at high data rates twoadjacent optical pulses or wave fronts may eventually overlap each otherdue to chromatic dispersion. Such overlapping can cause errors in datatransmission. The accumulation of chromatic dispersion increases as thedistance the optical signals travel increases. For low speed signals,dispersion is not typically a problem as they may use a smaller range ofwavelengths, resulting in a smaller range of delays in the transmissionpath, and they also have a longer time period in which to determine thestate of each bit.

Attempts to compensate for chromatic dispersion include the use ofdispersion compensating fibers, dispersion compensating opticalwaveguide gratings (e.g., fiber Bragg gratings), and a combination ofboth. Dispersion compensating fibers and dispersion compensating opticalwaveguide gratings introduce a negative chromatic dispersion with anequal and opposite sign to the accumulated dispersion in a fiber link.

Optical gratings suitable for dispersion compensation may include Bragggratings, and long period gratings. These gratings typically comprise abody of material with a plurality of spaced apart optical gratingelements disposed in the material. For example, a conventional Bragggrating comprises an optical fiber in which the index of refractionundergoes periodic perturbations along its length. The perturbations maybe equally spaced, as in the case of an unchirped grating, or may beunequally spaced as in the case of a chirped grating. That is, a chirpis a longitudinal variation in the grating period along the length ofthe grating. The wavelength reflected in a Bragg grating is directlyrelated to the period of the perturbation. Thus, in a chirped fibergrating, the reflected wavelength of the grating changes with theposition along the fiber grating. As the grating period increases ordecreases along a direction in the fiber grating, the reflectedwavelength increases or decreases accordingly. Therefore, differentspectral components in an optical signal are reflected back at differentlocations along the grating and accordingly have different delays. Suchwavelength dependent delays may be used to negate the accumulateddispersion of an optical signal. Chirped gratings may be linearlychirped (having perturbations that vary in a linear fashion),non-linearly chirped, or randomly chirped.

Fiber Bragg gratings reflect light over a given waveband centered arounda wavelength equal to twice the spacing between successiveperturbations. The reflected wavelength is given by the Bragg Equationλ=2 dn, where n is the effective index of the grating, λ is thereflected wavelength, and d is the distance between successiveperturbations. The remaining wavelengths pass essentially unimpeded. Theability to pass some wavelengths in an unimpeded manner is desirable inoptical filtering applications. In such applications, the frequency ofthe grating can be selected to reflect (i.e., filter) undesiredwavelengths, while allowing desired wavelengths to pass.

Fiber gratings may be extrinsically chirped or intrinsically chirped. Anextrinsic chirp refers to a chirp in the grating that is obtained byapplying an external perturbation generating field to the fiber. Forexample, to create an extrinsically chirped grating, an externalgradient, typically comprised of strain gradients or temperaturegradients, is applied along the length of a non-chirped fiber grating,resulting in non-uniform changes in properties of the fiber grating,thus creating a chirp. An extrinsic chirp is valuable in that it may beapplied to adjust the parameters of the grating, and it may be used tocontrol the chromatic dispersion of a fiber Bragg grating.

There are disadvantages, however, in forming chirped gratings with anexternal gradient. The maximum range of chirping that can be achieved islimited in that relatively large gradients or strains are required toobtain a range of chirping. Such externally applied strains may have anegative impact on the reliability of the fiber, such as by causing thefiber to fracture. Thus, the maximum chirp rate that can be imposed onthe grating is limited by the material properties of the fiber.

An intrinsic chirp refers to a chirp in the grating that has beenincorporated into the fiber during the fabrication process. For example,an intrinsic chirp may be achieved by using a phase mask in which theperiod of the phase mask varies in some manner. When radiation isapplied to the fiber through the phase mask (thus altering the index ofrefraction), the resulting fiber will be intrinsically chirped. Usingthis technique, broadband gratings may be produced which can compensatefor chromatic dispersion in multi-channel system. However, intrinsicchirp corrects a fixed amount of dispersion in a specified wavelengthspectrum. While intrinsically chirped gratings are useful incommunication systems where a specific amount of dispersion compensationis required, the dispersion and amplitude response of the grating isessentially fixed. Thus, intrinsically chirped gratings by themselvesare not well suited to situations in which dynamically adjustabledevices are required.

A variety of approaches exist for adjusting the spectral response (i.e.,“tuning”) of gratings in optical fibers. The application of strain tothe grating is one method. For example, simply stretching the grating bygripping the fiber on either end of the grating and then putting thefiber in tension. The act of imparting a tensile strain on the gratingresults in a proportional increase in the wavelength of the spectralresponse, as per the following equation${\Delta\;\lambda} = {{\lambda_{\max}\left( {1 - p_{e}} \right)}\frac{\Delta\; L}{L}}$where Δλ is the wavelength shift resulting from the imparted tensilestrain, ΔL/L; λ_(max) is the maximum wavelength; and p_(e) is the strainoptic coefficient of the grating (an intrinsic property of the fiber inwhich the grating is written). Other approaches use bending of a simplestructural member to put a strain on an attached grating. Each of theseapproaches is limited in the applications it can address.

In particular, a problem with current devices and methods requiring theapplication of tensile strain to the grating is the potential fortensile failure of the fiber. This problem severely limits the amount ofstrain that can be reliably imposed. Methods wherein the fiber isattached to a support member and the support member is then put intotension are likewise limited.

Short gratings (less than approximately 100 mm in length) have also beentuned using axial compression. For example, a grating may be attached tothe inside of a ferrule and the ferrule placed in compression. Whenusing a uniform ferrule, the wavelengths reflected will shift uniformlydownward. When using a non-uniform ferrule, both the dispersioncharacteristics of the grating and the center wavelength change withchanges in the force applied to the ferrule. Since only compression isused, tensile failure of the grating ceases to be an issue. However, thelength of grating that can be economically tuned using this method isvery limited. The length is also limited by column buckling of theferrule, which requires that for a given strain, the diameter of aferrule increases as a function of its length. Furthermore, the forcerequired to generate a given strain increases with the square of thediameter. These factors limit the use of the described compressiontuning methods for long gratings (greater than approximately 100 mm inlength).

Another problem current devices and methods for tuning share is that thewavelengths shift in all sections of the grating at the same time,although not necessarily by the same amount. Designs have been attemptedthat independently tune small elements within a grating. These designshave generally not been commercially successful due to their greatcomplexity, large number of parts and high cost. For example, U.S. Pat.No. 5,694,501 discloses an apparatus and method of controlling strain ina Bragg grating that requires a segmented piezoelectric stack withquasi-distributed voltage control. The segmented piezoelectric stack andcontrol system are complex, and a constant supply of power is requiredto maintain a selected strain profile.

Clearly, previous attempts to provide dispersion compensating devicesand methods have not produced adequate results. In particular, previousattempts have not satisfactorily addressed the unique issues associatedwith adjusting the spectral response of long gratings or sections oflong gratings. If methods representing the current technology were usedto adjust the spectral response of long gratings, the result would beseverely limited ranges of adjustability, or unacceptably high levels oftensile strain (>1%) on the fiber for high reliability. Currentlyavailable devices and methods also do not provide the capability to tunethe dispersion of a single channel (wavelength) of a multi-channelgrating at any wavelength within a broad operating range. A need existsfor an apparatus and method that addresses the above describedshortcomings.

SUMMARY

The embodiments according to the invention described herein provideapparatuses and methods for adjusting the spectral response of anoptical waveguide grating in a flexible manner.

Some embodiments according to the invention provide settable or tunabledispersion compensators. Other embodiments according to the inventionmay be used in tunable add/drop multiplexers or a variety of otherdevices that would benefit from the use of an adjustable filter. Stillother embodiments according to the invention utilize mechanicaladjustment of the spectral response such that once set the apparatus hasno power requirement. Still other embodiments according to the inventionprovide compressive preload on the optical waveguide grating to reduceor eliminate reliability concerns related to tensile failure. Stillother embodiments according to the invention allow selection andadjustment of any particular channel within the operating band of amulti-channel grating. Still other embodiments according to theinvention allow the wavelength to be set at each of two points in anoptical wavelength grating, with the result being a defined channelwidth and dispersion.

One embodiment according to the invention provides an apparatus foradjusting the spectral response of an optical waveguide grating. Theapparatus includes a support member having a centroidal axis. Thesupport member is configured for longitudinal bending and rotating aboutits centroidal axis. A longitudinal optical waveguide grating can beattached to the support member. The optical waveguide grating may beparallel to the centroidal axis, non-parallel to the centroidal axis, orwound about the centroidal axis. The spectral response of the opticalwaveguide grating is adjusted by bending and rotating the support memberabout its centroidal axis.

Another embodiment according to the invention provides a method foradjusting the spectral response of an optical waveguide grating. Themethod includes attaching a longitudinal optical waveguide grating to alongitudinal support member having a centroidal axis. The support memberis configured for longitudinal bending and rotating about the centroidalaxis. The support member is longitudinally bent and rotated about thecentroidal axis and thereby alters the spectral response of the opticalwaveguide grating.

As used herein, the neutral axis is the line or plane in a supportmember under transverse pressure, at which the support member is neitherstretched nor compressed (i.e., where the longitudinal stress is zero).

As used herein, the centroidal axis connects the centroids of anenvelope 538 encompassing the cross sections of the support member. Thesupport member is said to be straight or curved in accordance with theshape of its centroidal axis.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an exemplary optical fiber Bragg grating.

FIGS. 2A–2C illustrate one method of forming an apparatus for adjustingthe spectral response of an optical waveguide grating according to theinvention.

FIG. 3 illustrates the performance of an apparatus for adjusting thespectral response of an optical waveguide grating like that illustratedin FIGS. 2A–2C.

FIG. 4 illustrates another embodiment of an apparatus for adjusting thespectral response of an optical waveguide grating according to theinvention.

FIG. 5 illustrates yet another embodiment of an apparatus for adjustingthe spectral response of an optical waveguide grating according to theinvention.

FIG. 6 illustrates an alternate configuration for the apparatus of FIG.5.

FIG. 7 illustrates yet another embodiment of an apparatus for adjustingthe spectral response of an optical waveguide grating according to theinvention.

FIG. 8 illustrates the adjustment of the spectral response of a Bragggrating using the apparatus FIG. 7.

FIG. 9 illustrates a support member that is asymmetric about its neutralaxis.

FIG. 10 illustrates one method of tuning the spectral response of anoptical waveguide grating or section thereof according to the invention.

FIG. 11 illustrates uniformly shifting the spectral response of anoptical waveguide grating.

FIG. 12 illustrates altering the chromatic dispersion of an opticalwaveguide grating.

FIG. 13 illustrates altering the center wavelength and the chromaticdispersion of an optical waveguide grating.

FIG. 14 illustrates an embodiment of an apparatus for adjusting thespectral response of an optical waveguide grating that is adjusted bybending and rotating the support member according to the invention.

FIG. 15 is a cross sectional view along line 15—15 of the apparatus ofFIG. 14.

FIG. 16 illustrates yet another embodiment of an apparatus for adjustingthe spectral response of an optical waveguide grating that is adjustedby bending and rotating the support member according to the invention.

FIG. 17 illustrates yet another embodiment of an apparatus for adjustingthe spectral response of an optical waveguide grating that is adjustedby bending and rotating the support member according to the invention.

FIGS. 18A and 18B illustrate additional embodiments of an apparatus foradjusting the spectral response of an optical waveguide grating that areadjusted by bending and rotating the support member according to theinvention.

FIGS. 19A–19E illustrate the delay slope and dispersion curves of theapparatuses of FIGS. 18A and 18B.

FIGS. 20A and 20B schematically illustrate an embodiment of a mechanismfor bending and rotating the support member of the apparatuses of FIGS.14–18B according to the invention.

FIG. 21 schematically illustrates another embodiment of a mechanism forbending and rotating the support members of the apparatuses of FIGS.14–18B according to the invention.

DETAILED DESCRIPTION

In the following detailed description, reference is made to theaccompanying drawings, which form a part hereof, and in which is shownby way of illustration specific embodiments in which the invention maybe practiced. It is to be understood that other embodiments may beutilized and structural or logical changes may be made without departingfrom the scope of the present invention. The following detaileddescription, therefore, is not to be taken in a limiting sense, and thescope of the present invention is defined by the appended claims.

For reasons of clarity, the various embodiments according to theinvention are described herein with respect to optical waveguidesconfigured as optical fibers. However, the teachings of the inventionare equally applicable to other types of optical waveguides. Similarly,the various embodiments according to the invention are for the most partdescribed herein with respect to devices and methods that adjust thespectral response of portions of long gratings. However, the embodimentsaccording to the invention are applicable to adjusting the spectralresponse of a portion of any length grating, including the full lengthof a grating. Long gratings are generally considered to be gratingshaving a length of 100 mm or greater. Broadband gratings in the range of1–3 meters long (or even longer) may be used in long distance opticalcommunication systems.

Referring now to the drawings and in particular to FIG. 1, an opticalwaveguide 10 is shown to be configured as an optical fiber of which onlya relatively short longitudinal length is depicted and which includes afiber core 12 and a fiber cladding 14 surrounding the fiber core 12. Thefiber core incorporates a grating 16 that includes a plurality ofgrating elements 18, each grating element 18 extending substantiallynormal to the longitudinal axis of the core 12. The spectral response(i.e., wavelength and chromatic dispersion) of the grating 16 can beadjusted (or “tuned”) by altering the average index of refraction of thegrating structure, changing the spacing between grating elements 18, orby using a combination of the two.

Axial Pre-Compression

One aspect of the present invention provides an apparatus for adjustingthe spectral response of an optical waveguide grating having highreliability (i.e., reduced susceptibility to fiber failure due to staticfatigue) by applying a compressive axial preload to the grating prior topackaging the grating. This compressive preload reduces or eliminatespotentially damaging tensile strain in the grating when the spectralresponse of the grating is subsequently adjusted using a controlledapplication of tensile strain.

One embodiment of an apparatus for adjusting the spectral response of anoptical waveguide grating having a compressive preload and a method offorming such an apparatus is illustrated in FIGS. 2A–2C. The apparatushas an optical waveguide 10, including an optical grating 16, attachedto a support member 30. The optical grating 16 is subjected to axialcompression by support member 30 so that optical grating 16 is under acompressive axial strain prior to any adjustment (i.e., “tuning”) of thespectral response of the grating. The apparatus is tuned by altering thestrain in the optical grating 16. In one embodiment, the strain ofoptical grating 16 is altered by the application of mechanical bendingforces to the support member 30. In another embodiment, the strain ofoptical grating 16 is altered by controlled heating or cooling of thesupport member 30.

One method of forming an apparatus for adjusting the spectral responseof an optical waveguide grating having a compressive preload is asfollows. First, the range of wavelengths (operating band) of a singlechannel or channels to be adjusted is determined, together with therange of dispersion values (dispersion range) over which the gratingwill be adjusted. Next, a preliminary optical grating design that allowsthe optical grating to be adjusted over the desired dispersion range andoperating band is determined based on the desired use. Next, the maximumwavelength increase and associated maximum tensile strain required totune the preliminary optical grating across the desired operating rangeis determined. Next, a final optical grating is selected, wherein ateach location in the final optical grating the reflected wavelength islonger than the wavelength in the preliminary optical grating design bythe previously determined maximum wavelength increase.

After the final optical grating is selected using the above steps, asupport member 30, such as a straight beam having a uniformcross-section, is provided. While still separated from the opticalwaveguide 10 having the selected final optical grating 16, a supportsurface 32 of support member 30 is elongated in a controlled manner(FIG. 2A) as indicated by arrows 38. Support surface 32 may be elongatedin a controlled manner by imparting a controlled tensile strain tosupport member 30. A controlled tensile strain may be imparted by theapplication of mechanical force to support member 30, such as by bendingthe support member 30 in a controlled arc having a constant bend radiusby the application of a uniform bending moment across the length of thesupport member 30. Alternately, a controlled tensile strain may beimparted by stretching the support member 30. Alternately, supportsurface 32 may be elongated in a controlled manner by applying acontrolled temperature change to the support member 30, so long assupport member 30 has a non-zero coefficient of thermal expansion.

After support surface 32 of support member 30 is elongated by apredetermined distance (either by the application of controlled strainor the application of controlled heating/cooling), the substantiallyunstrained optical waveguide 10 and its associated grating 16 issecurely attached along the entire length of the grating to theelongated support surface 32 of support member 30, such that no relativemovement between the support surface 32 and the grating 16 is permitted(FIG. 2B). The optical waveguide grating 16 is attached to the elongatedsupport surface 32 of the support member 30 such that the longitudinalaxis of the grating 16 is in alignment with the direction of thecontrolled elongation in the support surface 32. The optical waveguidegrating 16 may be attached to the support member 30 in a variety ofacceptable manners, for example, with adhesive, solder or mechanicalclamping devices.

After the optical waveguide 10 and associated grating 16 are securelyattached to the surface 32 of support member 30, the mechanical orthermal conditions causing elongation of the support surface 32 areremoved. As support surface 32 returns to its shorter, pre-elongatedcondition, a compressive axial strain (the compressive preload)indicated by arrows 40 is applied to the attached optical waveguide 10and associated grating 16 (FIG. 2C).

The spectral response (wavelength and/or chromatic dispersion) of theaxially compressed grating 16 may subsequently be adjusted (i.e.,“tuned”) by application of a strain and/or temperature distribution onthe support member 30 along the length of the grating 16. Theapplication of a strain and/or temperature distribution on the supportmember 30 along the length of the grating 16 imparts a correspondingtensile strain in the grating 16, which alters both the index ofrefraction of the grating 16 and the spacing of grating elements 18. Thecontrolled application of a strain and/or temperature distribution thusallows the wavelength and/or the chromatic dispersion characteristics ofthe grating 16 to be adjusted to achieve a desired spectral response.

In one embodiment according to the invention, the controlled extensionof support surface 32 is sufficient to cause the magnitude of thecompressive axial strain on the grating 16 to be equal to or greaterthan a previously determined maximum tensile strain required to achievethe maximum wavelength change in the grating 16.

In an alternate embodiment according to the invention, the supportmember 30 is made from a material having a coefficient of thermalexpansion (CTE) different from the CTE of the optical waveguide 10 andassociated grating 16. Instead of applying a controlled strain toelongate surface 32 of the support member 30 (such as by bending orstretching the support member 30 as described above), a thermal load isapplied to the support member. The support member 30 is heated (orcooled) by the thermal load to a temperature at which the differentialexpansion of the support member 30 and the grating 16 results in acompressive axial strain on the grating 16 when the thermal load isremoved. In a preferred embodiment, the magnitude of the compressiveaxial strain is equal to or greater than the maximum tensile stress thatmight occur over the operating temperature range of the apparatus inorder to achieve the maximum wavelength change.

Thus, in a preferred method of adjusting the spectral response of thegrating 16, the compressive axial strain (the compressive preload)applied to the grating 16 prior to tuning has a magnitude equal to orgreater than the magnitude of a maximum tensile strain imparted to thegrating 16 during subsequent tuning operations. In this manner, duringsubsequent tuning, the optical waveguide 10 and grating 16 do notexperience axial tension, and therefore are not subject to tensilefailure.

An exemplary application of the method for adjusting the spectralresponse of a grating is as follows. First, assume a glass gratinglength of 100 mm and a round trip delay in the glass grating of 10picosec/mm (delay=2×speed of light×effective refractive index). Thus,the maximum delay (from the near end of the grating to the far end ofthe grating and back to the near end) is 1000 ps. Next, as illustratedin the graph of FIG. 3, assume that the grating will be tuned around aconstant center wavelength of 1554 nm, the maximum desired dispersion is500 ps/nm, and the minimum desired dispersion is 125 ps/nm. Then, themaximum tensile strain required to tune the grating across the desiredoperating range is determined by the equation:${{required}\mspace{20mu}{strain}} = \frac{\Delta\;\lambda}{\lambda_{\max}\left( {1 - p_{e}} \right)}$Thus, the Required Strain=1.5/[1554*(1−0.24)]=0.13%. Therefore, a 0.13%tensile strain is applied to the support beam prior to attachment of thegrating, either by simple tension or bending of the support member.After attaching the grating, the tensile strain is removed from thesupport member, with the result being an axially compressive force andthe elimination of tension in the waveguide. This example assumes themechanical strength of the grating is negligible relative to thestrength of the support beam.Bending Moments

Another aspect of the present invention provides an apparatus and methodfor adjusting the spectral response of an optical grating through theapplication of a pair of bending moments. The pair of bending momentsco-operate to create a controlled bending moment gradient over a sectionof a support member to which the optical grating is attached. Apparatusand methods that embody this aspect of the invention are useful formechanically tuning optical waveguide gratings of any length, or aselected portion of any length of optical waveguide grating. Because theapparatus and method can tune a selected portion of an optical waveguidegrating, they are beneficially used for tuning a selected channel orchannels in a multi-channel grating. Specifically, they are beneficiallyused in a broadband wavelength division multiplexed (WDM) optical systemhaving a plurality of optical wavelength channels.

One embodiment of an apparatus 100 for adjusting the spectral responseof an optical waveguide grating is illustrated in FIG. 4. The apparatushas attached to it a broadband optical grating 116 having an operatingbandwidth encompassing a plurality of channels (wavelengths). Theoptical grating 116 may be chirped, and may be of any suitable type ofgrating. For example, the grating may be a fiber Bragg grating, or along period grating. The grating 116 is attached to a support member 130(which is a flat beam in this illustration). A bending moment applicator140 is configured to apply two bending moments to the support member130. The grating 116 and support member 130 are moveable with respect tobending moment applicator 140. The bending moment applicator 140includes a base 142 having two rotatable discs 144, 146 attachedthereto. Each disc 144, 146 has two pairs of pins 150 (for a total offour pins 150 on each disc 144, 146) projecting therefrom, in adirection normal to the plane of rotation of the discs 144, 146. Thesupport member 130 and attached grating 116 are positioned between eachpair of pins 150, such that rotation of the discs 144, 146 causes onepin 150 of each pair of pins to apply a force to the support member 130.A thumbwheel 160 and worm gear 162 are associated with each disc 144,146. As either of the thumbwheels 160 are rotated, the worm gear 162causes the associated disc 144, 146 to rotate. As the discs 144, 146rotates, forces are applied to the support member 130 via pins 150,thereby creating a bending moment gradient in support member 130 betweenthe discs 144, 146. In the embodiment illustrated in FIG. 4, both theright and left discs 144, 146, respectively, have been rotatedclockwise. In this case, the grating 116 sees a linear strain gradientbetween the discs 144, 146. The maximum axial compression on the grating116 is adjacent to the downward pushing right pin of the left disc 144and the maximum tension is adjacent to the upward pushing left pin onthe right disc 146. Although eight pins 150 are shown on the two discs,it should be noted that at any point in time only four of the pins 150are applying force to the beam 130.

In one embodiment according to the invention, the bending momentapplicator 140 is configured to apply the two bending moments to thesupport member 130 separated by a distance less than the length of theoptical grating 116. The distance separating the application points ofthe bending moments may be selected so that only a single channel of theplurality of channels, or a subset of the plurality of channels, in theoptical grating 116 are tuned. In this embodiment, the bending momentapplicator 140 is preferably moveable with respect to the support member130 and optical grating 116, such that the application points (of thebending moments can be located over any desired channel(s) of thebroadband optical grating 116.

In another embodiment according to the invention, the bending momentapplicator 140 is configured to apply the two bending moments to thesupport member separated by a distance equal to or greater than thelength of the optical grating 116. In this configuration, the bendingmoments can be adjusted to tune the spectral response of the entiregrating 116.

Embodiments of the invention may employ axial pre-compression of theoptical grating in addition to the application of a pair of bendingmoments to the support member. As described above, the optical grating116 may be attached to the support member 130 such that the supportmember 130 axially compresses the optical grating 116. Preferably, thecompressive axial strain (the compressive preload) applied to theoptical grating 116 has a magnitude equal to or greater than themagnitude of a maximum tensile strain imparted to the grating duringsubsequent tuning operations using the application of a pair of bendingmoments.

FIG. 5 illustrates another embodiment of the present invention, anapparatus 200 for adjusting the spectral response of an opticalwaveguide grating. In the apparatus 200 of FIG. 5, the relationshipbetween the two bending moments is fixed so that the dispersion of anoptical waveguide grating 216 is changed, but not the center wavelength.The apparatus of FIG. 5 includes the grating 216 attached to a flat beamsupport member 230. The optical grating 216 and support member 230 aremoveable with respect a bending moment applicator 240. The bendingmoment applicator 240 includes a housing 242 having guide members 244adjacent both sides of the housing 242. The support member 230 andattached grating 216 extend across the housing 242 and are positionedbetween the guide members 244. To apply a moment to the grating 216, athumbwheel 250 turns a lead screw 252, which in turn moves a cam 260 tothe left or right of its normal centered location in the housing 242.The cam's angled cam surfaces 262 contact a movable central part 264,which rotates about a pivot point 266. Pins 270 mounted symmetricallyabout the pivot point 266 provide bending forces to the flat supportbeam 230. In one embodiment, a portion of the cam 260 is exposed throughthe top surface 276 of the housing 242, so that the position of the cam260 relative to the housing 242 can be used to determine the currenttuning amount.

FIG. 6 illustrates apparatus 200 of FIG. 5 slidably mounted on rails 302of an assembly 300. Assembly 300 includes a long grating 316 on asupport beam 330. In use, the apparatus 200 is moved to a positioncentered over a selected point in the grating that reflects a knownwavelength that becomes the center wavelength of the tuned region. Theapparatus 200 is then secured in position on rails 302 of assembly 300and the chromatic dispersion of the tuned region immediately around theselected center wavelength is adjusted using the thumbwheel 250.

FIG. 7 illustrates a test jig 400 for applying bending moments to asupporting beam 430. Test jig 400 includes guide pins 450 mounted onstationary base 451, and a rotating disk 452 mounted on base 451.Application of a force, F, to the end of the tuning rod 440 causes disk452 to rotate. Rotating disk 452 includes pins 453, 454, 455, 456 toapply bending moments to beam 430 and the grating 416 thereon. Thebending moments created by the test jig 400 create strain profilesidentical to the apparatus 200 of FIG. 5.

FIG. 8 illustrates the chromatic dispersion tuning of a Bragg gratingwritten at 430 ps/nm and adjusted using the test jig 400 shown in FIG.7. The six lines represent six different magnitudes of force applied tothe end of the tuning rod 440. As can be seen, the dispersion (slope ofthe delay curves) around the central wavelength is effectively adjustedusing this method. In this example, the chromatic dispersion wasadjusted down to a minimum of 130 ps/nm, and up to a maximum of 1040ps/nm. In the variously described embodiments according to theinvention, the support member 130, 230, 330 may optionally be formed ofa material having a coefficient of thermal expansion (CTE) belowapproximately 5 ppm/C°. A low CTE minimizes changes in the spectralresponse of a grating due to thermal conditions, and is desirable wherethe device is not intended to be thermally responsive. For example, thesupport member 130 may be formed of graphite, graphite compositematerials, metal alloy materials, especially nickel alloy materials, andthe like. Suitable nickel alloy materials include those available underthe trade names INVAR (36% nickel, about 63% iron, and less than 1%total of manganese, silicon, and carbon) and KOVAR (29% nickel, 17%cobalt, about 53.5% iron, and about 0.5% total of manganese, silicon,and carbon.

Although the support members 130, 230, 330 described so far havesymmetric cross-sections, the support member may alternately beasymmetric about its neutral axis. For example, a support member 530illustrated in FIG. 9 is asymmetric about its neutral axis 532. Theoptical grating 536 is attached to the asymmetric support member 530 ata region most distant from the neutral axis 532. As used herein, theneutral axis is the line or plane in a support member under transversepressure, at which the support member is neither stretched norcompressed (i.e., where the longitudinal stress is zero). Depending uponthe shape (cross-section) of the support member, the neutral axis maybe, but is not necessarily, colinear with the centroidal axis 552. Asused herein, the centroidal axis 552 connects the centroids of anenvelope 538 encompassing the cross sections of the support member. Thesupport member is said to be straight or curved in accordance with theshape of its centroidal axis. An asymmetric support member shape asshown in FIG. 9 has the grating 536 placed at a relatively largedistance from the neutral axis 532, as compared to the size of grating536. When a vertical bending moment is applied, this large distanceprovides a larger ratio of strain per unit bending moment than symmetricsupport member designs. The asymmetric support member shape also resultsin a lower average strain on the support member material than asymmetric support member shape.

A method of tuning the spectral response of an optical grating orsection thereof according to the invention is illustrated in FIG. 10.First, the optical grating is attached to support member (step 500). Thegrating may optionally be attached to the support member such that thesupport member axially compresses the grating, as described above. Thegrating is attached to the support member along its length, typicallyparallel to and spaced from a centroidal axis 552 of the support member.The support member preferably has a uniform cross-section along thelength of the optical grating.

Next, a portion of the grating that will have its spectral responseadjusted or “tuned” is selected (step 502). Typically, the selectedportion of the grating will be centered about a single channel orwavelength to be adjusted. However, the selected portion may encompass aplurality of channels, and may include the entire length of the grating.

The support member is next deformed by applying a pair of bendingmoments (operating on the support member) at some distance from eachother (step 504). The location of the bending moments is selected suchthat one bending moment is applied adjacent to one end of the adjustedregion and the other bending moment is applied adjacent the other end ofthe adjusted region. Application of the bending moments on the supportmember creates an axial strain profile along the length of grating,thereby altering the spectral response of the grating within theadjusted region.

As the bending moments are applied, the spectral response of the gratingcan be monitored. The magnitude and location of one or more of thebending moments can then be adjusted independently and dynamically totune the spectral response of the optical grating until a desiredspectral response is achieved (step 506).

For example, assume the objective is to tune a uniformly chirped gratingto provide −1300 ps/nm dispersion in the 1 nm band from 1550 nm to 1551nm. The two bending moments could be spaced apart by a distance ofapproximately 130 mm. The moment at one end of the band would then beadjusted to result in the grating located at that position reflectingthe 1551 nm wavelength and the moment at the other end of the band wouldthen be adjusted so that the grating would reflect the 1550 nmwavelength at that position. That is, the wavelength is set at each oftwo points in the grating, with the result being a defined channel widthand dispersion. In actual practice, the tuned band would typicallyinclude a safety margin and be somewhat larger and than the band of theoptical signal.

Various combinations of bending moments and the resulting change in thespectral response of the grating are illustrated in FIGS. 11–13. In eachof FIGS. 11–13, the “original” spectral response is shown as solid line“A”, while the “adjusted” spectral response is shown as dashed line “B”.Depending upon how the bending moments are applied, altering thespectral response of the optical grating may include uniformly shiftingspectral response (i.e., center wavelength and chromatic dispersion) ofthe grating (FIG. 11), altering the chromatic dispersion of the grating(FIG. 12), or shifting the center wavelength and also altering thechromatic dispersion of the optical grating (FIG. 13).

In each of the illustrated examples, a chirped grating is attached tothe top surface of a support beam, the short wavelength end of thegrating is to the left side of the diagram and light is launched intothe grating from the right side of the diagram. In each case the delayvs. wavelength chart represents only the length of grating between theinnermost force application points.

FIG. 11 illustrates uniformly shifting the spectral response of thegrating. Specifically, the center wavelength and dispersion of thegrating has shifted as a result of four equal forces creating equal andopposite moments near the ends the support beam.

FIG. 12 illustrates altering or tuning the chromatic dispersion of thegrating. The four forces are configured so that the moment created bythe inside forces is equal and opposite to the moment created by theoutside forces. This results in a uniform moment gradient between theinner two force application points and results in a change in thechromatic dispersion of the grating, while the center wavelength staysthe same.

FIG. 13 illustrates a combination of wavelength shift and chromaticdispersion tuning achieved by superimposing the examples of FIGS. 10 and11. In this superposition of forces, the clockwise and counterclockwisemoments at the right end of the beam tend to cancel resulting in only asmall upward force. The magnitude and direction of the bending moment ateach location along the grating are proportional to the wavelength shiftat that location.

As noted above, the “two-moment” apparatus and method disclosed hereincan be used to tune the spectral response of an optical waveguidegrating over any portion of the grating, including the full length of agrating. However, for a long grating, the change in spectral responseper unit strain would be small. Achieving a meaningful or useful changein the spectral response would require a large strain on the fiber. Inmany applications, it is beneficial to tune a single channel within amulti-channel grating, or independently tune two or more differentchannels within a long multi-channel grating. This objective can beeasily achieved using the apparatus and method disclosed herein, becauseadjusting the spectral response of only a portion of a grating requiresa lower average/peak strain than needed to tune the full length of thegrating.

Bend and Rotate

Another aspect of the present invention provides an apparatus and methodfor adjusting the spectral response of an optical grating by attachingthe grating to a longitudinal support member, applying a bending momentto the support member to create a bend or curve in the support member,and then rotating the support member about its centroidal axis. As thesupport member is rotated, depending upon its initial position on thesupport member, the grating moves from an area of tension on the outsideof the support member bend to an area of compression on the inside ofthe support member bend (thus reflecting a shorter wavelength), oralternately move from the inside of the support member bend outward (andthus reflecting longer wavelengths).

One exemplary embodiment of an apparatus for adjusting the spectralresponse of an optical waveguide grating according to the invention isillustrated in FIG. 14. A strain-imparting support member 550 (in thisexemplary embodiment a longitudinal rod having a circular cross-section)is configured for simultaneous longitudinal bending and rotating aboutits centroidal axis 552. A longitudinal optical waveguide 554 containinga grating 556 is attached along its entire length to thestrain-imparting support member 550. In one embodiment according to theinvention, the grating 556 is a long optical grating, and has a lengthof at least 100 mm. In another embodiment according to the invention,the long optical grating has a length of at least 1 m.

The support member 550 is operatively coupled to a mechanism (not shown)for bending and rotating the support member. The mechanism applies abending moment 560 to the support member 550 for longitudinally bendingthe support member. The support member 550 is then rotated (as indicatedby arrow 562) about its now curved or bent centroidal axis 552 to adjustthe center wavelength of the grating 556.

In one embodiment according to the invention, the support member 550bends and/or rotates as a function of temperature, such that theapparatus is suitable for use as a thermal compensation package. Thethermally responsive bending and/or rotating may be accomplished byselecting the material of the support member 550 to be responsive totemperature changes, or by configuring the mechanism for bending androtating to be responsive to temperature changes, or a combination ofboth.

The grating 556 and associated waveguide 554 may be attached to thesupport member 550 in several different orientations. In one embodimentaccording to the invention, as illustrated in FIG. 14, the longitudinalaxis of the grating 556 is parallel to the centroidal axis 552 of thesupport member 550. In this orientation, the mechanism for bending androtating the support member 550 about its centroidal axis 552 tunes thewavelength of the optical grating 556 by controlling the longitudinalbending and rotating of the strain-imparting support member inaccordance with the equation:λ=λ_(o)[1+(1−P)r cos(θ)/R]Where:

-   -   λ_(o)=unstrained wavelength of the grating;    -   P=the strain optic coefficient of the grating;    -   R=radius of the bend in the support member;    -   r=radial distance from the centroidal axis to the core of the        grating; and    -   θ=angle of rotation about the centroidal axis (as shown in FIG.        15).

In another embodiment according to the invention, as illustrated in FIG.16, the support member 550 has a cavity 570 extending longitudinallytherethrough. The grating 556 is positioned within the cavity 570 in thesupport member. In the embodiment of FIG. 16, the cavity 570 extendingthrough the support member 550 is parallel to the centroidal axis 552 ofthe support member. In that instance, the mechanism for longitudinallybending and rotating the tubular support member 550 about its centroidalaxis 552 tunes the wavelength of the optical grating 556 by controllingthe longitudinal bending and rotating of the support member inaccordance with the equation provided above.

In another embodiment according to the invention, as illustrated in FIG.17, the support member 550 has a cavity 570′ extending longitudinallytherethrough that intersects the centroidal axis 552 of the supportmember and/or is not parallel to the centroidal axis 552 of the supportmember. Support member 550 is shown in an unbent condition, so that therelationship between cavity 570′ and centroidal axis 552 may be clearlyseen. The longitudinal axis of the grating 556 is positioned within thelongitudinal cavity 570′ in the support member. Because the longitudinalcavity 570′ of the support member 550 is no longer parallel to thecentroidal axis 552 of the support member, the result of bending androtating support member 550 is not a simple wavelength shift in thespectral response of the grating. The mechanism for longitudinallybending and rotating the support member about its centroidal axis 552tunes the wavelength of the optical grating 556 by controlling thelongitudinal bending and rotating of the support member in accordancewith the equation provided above. However, the value of r is equal tothe distance between the centroidal axis 552 of the support member 550and the grating 556, and changes as a function of location along thegrating 556. In the case shown where the grating crosses the centroidalaxis of the support member, subsequent bending and rotation does notchange the wavelength at the crossing point, but changes the dispersionover the length of the grating.

In another embodiment according to the invention, the longitudinal axisof the grating 556 is wound about the centroidal axis 552 of the supportmember 550. In the exemplary embodiment of FIG. 18A, an opticalwaveguide 554 having an optical grating 556 is helically wound about thecentroidal axis 552 of a longitudinal support member 550 having acircular cross-section. In this orientation between grating 556 andcentroidal axis 552, the result of bending and rotating support member550 is not a simple wavelength shift. Instead, when the support member550 is bent, some sections of the grating 556 are placed in tension (onthe “outside” of the bend) and other areas of the grating 556 are placedin compression (on the “inside” of the bend). The spectral response isthus modulated in a manner that correlates to the helix angle (α) of thegrating 556 as it winds around support member 550. The mechanism forbending and rotating the support member 550 about its centroidal axis552 tunes the wavelength of the optical grating 556 by controlling thelongitudinal bending and rotating of the strain-imparting support memberin accordance with the equation provided above. However, the angle ofrotation (θ) becomes a function of location on the grating 556 and willtherefore vary for each point along the grating.

As an example, assume that the unstrained grating 556 of FIG. 18A has auniform dispersion (i.e., the delay slope is constant) before thesupport member 550 is bent. Then, it can be seen in the graph of FIG.19A that bending the support member 550 results in a modulation aroundthe initial (unstrained) delay line. Since dispersion is defined as theslope of the delay line, it is clear that the dispersion changes as afunction of the position in the grating 556. The graph of FIG. 19B showsthe dispersion of the grating 556 as a function of wavelength andrelates to the strain distribution used to generate the graph of FIG.19A. By rotating the support member 550 without changing the radius ofthe bend, the dispersion at any wavelength will vary between theextremes shown in FIG. 19B. Thus a single wavelength or narrow bandwithin the bandwidth of the grating 556 may readily have its dispersionadjusted. This embodiment thus provides the ability to tune only aselected section of a grating 556.

To avoid the creation of etalons in an optical waveguide grating (as istypically preferred in a grating-based dispersion compensation filter),the intrinsic chirp of the grating should be greater than the straininduced chirp caused by bending of the support member. This can beachieved by assuring that the following relationship is maintained:Intrinsic Chirp>λ_(o)*(1−P)*sin(α)/RWhere:

-   -   λ_(o)=unstrained wavelength of the grating;    -   P=the strain optic coefficient of the grating;    -   α=the helix angle (the angle between the fiber grating and the        axis of the support member);    -   R=radius of the bend in the support member.

For example, assume a grating written with a dispersion of 500 ps/nm hasan intrinsic chirp of about 20 nm of wavelength shift per meter ofgrating length (intrinsic chirp=20 nm/m). Further assume a maximumwavelength of approximately 1600 nm (λ_(o)=1600 nm). The strain opticcoefficient of a silica fiber is approximately 0.24 (P=0.24). If onedesires to bend the support member in an arc having a radius of 2 meters(R=2 m), then using the relationship given above, the helix angle αshould not exceed about 1.8 degrees.

If the bend in the support member has a larger radius, a larger helixangle may be used. For example, using the same grating as in the exampleabove, and a bend radius of 5 meters, the helix angle should not exceedabout 4.6 degrees.

The effect of the bend radius R on delay and dispersion is illustratedin FIGS. 19C and 19D for a grating helically wrapped about a supportmember. The plots of FIGS. 19C and 19D model an optical waveguidegrating having a dispersion of 500 ps/nm and an intrinsic chirp of about20 nm of wavelength shift per meter of grating length (intrinsicchirp=20 nm/m) helically wrapped around a support rod having a radius of7 mm. The grating is wrapped with a helix angle α of 2 degrees. Thestrain optic coefficient of the grating is approximately 0.24 (P=0.24).The support member is variously bent in an arc having a radius R of 5meters (curve S1), 10 meters (curve S2), and 20 meters (curve S3).

In the exemplary embodiment of FIG. 18A, a single optical waveguidegrating 556 is wound about the centroidal axis 552 of the support member550. The slope of the dispersion curve of the grating 516 (asillustrated in FIG. 19B) can be seen to vary continuously between theextremes, such that there is no region where the slope of the dispersioncurve is relatively constant. As a result, for a given rotation of thesupport member about its centroidal axis, at some points in the grating(i.e., where the slope of the dispersion curve is small) only a smallchange in dispersion will result, while at other points in the grating(i.e., where the slope of the dispersion curve is large) a large changein dispersion will result. In some applications, it is desirable thatthe slope of the dispersion curve be relatively constant, so that for agiven rotation of the support member about its centroidal axis, thechange in dispersion is approximately the same for all points along thegrating.

In an embodiment illustrated in FIG. 18B, two identical opticalwaveguide gratings 556, 556′ are wound about the support member 550.Gratings 556, 556′ are helically wrapped in a similar manner, with thegratings 556, 556′ mounted on the support rod such that the two gratingsare located on opposite sides of the support member (i.e. 180 degreesapart) at any point along their length. The optical signal for grating556 is launched into the short wavelength end (right) and the opticalsignal is launched into grating 556′ using the long wavelength end(left), such that their dispersion curves are of opposite signs and 180°apart. As illustrated in FIG. 19E, summing the dispersion curves ofindividual gratings 556, 556′ provides a dispersion curve having arelatively constant slope. Advantageously for some applications, thedispersion curve resulting from the summed individual dispersion curvescrosses the zero axis, such that the apparatus may be used to provideeither positive or negative dispersion, as needed.

As discussed above and as shown in FIGS. 14–18B, the grating 556 may beattached to the support member 550 such that the longitudinal axis ofthe grating 556 has several different possible orientations with respectto the centroidal axis 552 of the support member. Tuning of the grating556 is controlled in accordance with the equations provided above.

In another method for adjusting the spectral response of an opticalgrating 556 according to the invention, a generally unstrained grating556 is attached to an already strained (i.e., bent or flexed) supportmember 550. The grating 556 is attached to the section of the supportmember having the greatest tension (e.g., the outside of the bend). Asthe support member 550 is rotated about its centroidal axis 552, thegrating 556 moves from its original unstrained position on the outsideof the support member 550 bend to a position where it sees a compressivestrain as it moves toward the inside of the support member bend. Thegrating 556 is thus tuned using axial strain only, and no tensile strainis place on the grating 556.

The support member 550 has so far been described herein as having acircular cross-section. However, other cross-sectional profiles ofsupport member 550 may also be implemented. For example, support member550 may have an elliptical cross-section or any other non-circularcross-section. Additionally, the cross-section of support member 550 mayvary along the length of support member 550.

The mechanism for longitudinally bending and rotating the support memberand attached grating 556 about the centroidal axis 552 may comprise anysuitable device that provides controlled bending and rotating of thesupport member. One such mechanism 600 is illustrated in FIGS. 20A and20B, in which a positioning plate 602 includes at least one set ofpositioning grooves 604. Grooves 604 are sized to receive support member550, and are oriented to maintain a predetermined bend radius in supportmember 550. Multiple sets of grooves (for example, grooves 604′, 604″)corresponding to different bend radii may additionally be provided sothat support member 550 can be bent across a range of predetermined bendradii. Support member 550 may be rotated about its centroidal axis 552by, for example, rotating support member by hand, or by operablycoupling support member 550 to a drive motor 608. Drive motor 608 may bemanually controlled, or alternately controlled by controller 610 toautomatically rotate support member 550 and thereby adjust the spectralresponse of grating 556.

Another mechanism 620 for longitudinally bending and rotating thesupport member 550 and attached grating 556 about the centroidal axis552 is illustrated in FIG. 21. Support member 550 is held at its ends inferrules 622. Ferrules 622 are pivotable with respect to base member 624(as indicated by arrows 626), and are configured such that the distancebetween ferrules 622 may be altered (as indicated by arrows 627),thereby changing the bend radius of support member 550. For example,ferrules 622 may be moved between fixed, incremental positions in basemember 624. Alternately, the distance between ferrules 622 may becontinuously adjusted, such as by attaching one or both of ferrules 622to a drive mechanism 628, such as a screw drive, that moves ferrules 622closer together or further apart as necessary to change the bend radiusof support member 550. Drive mechanism 628 may be operated andcontrolled manually, or alternately controlled by a drive motor 630 andcontroller 632. Support member 550 may be rotated about its centroidalaxis 552 by, for example, rotating support member within ferrules 622 byhand, or by operably coupling support member 550 to a drive motor 634.Drive motor 634 may be manually controlled, or alternately controlled bycontroller 632 to automatically rotate support member 550 and therebyadjust the spectral response of grating 556.

The above-described embodiments according to the invention providesimple and cost effective ways to adjust (i.e., “tune”) the wavelengthand/or chromatic dispersion characteristics (i.e., the “spectralresponse”) of an optical waveguide grating, such as a fiber Bragggrating or long period grating. The various embodiments require a smallnumber of relatively simple parts, as compared to the devices andmethods previously available. The various embodiments according to theinvention provide apparatuses and methods for achieving a dispersioncompensation apparatus having high reliability (i.e., reducedsusceptibility to fiber failure due to static fatigue) by using acombination of beam bending and/or compressive preload on the opticalgrating to reduce tensile strain in the grating. Reduced strain in thegrating permits a wider bandwidth and tuning range.

Although specific embodiments have been illustrated and describedherein, it will be appreciated by those of ordinary skill in the artthat a wide variety of alternate and/or equivalent implementations andembodiments that achieve the same purposes may be substituted for thespecific embodiments shown and described without departing from thescope of the present invention. Those with skill in the optical,mechanical, and opto-mechanical arts will readily appreciate that thepresent invention may be implemented in a very wide variety ofembodiments. This application is intended to cover any adaptations orvariations of the embodiments discussed herein.

1. An apparatus for adjusting the spectral response of an opticalgrating in an optical waveguide, the apparatus comprising: a supportmember to which a longitudinal optical waveguide grating can beattached, the support member having a centroidal axis and beingconfigured for longitudinal bending and rotating about the centroidalaxis, and a mechanism for longitudinally bending and rotating thesupport member about the centroidal axis, wherein the mechanism forlongitudinally bending and rotating the support member about thecentroidal axis alters the wavelength of the optical waveguide gratingby controlling the longitudinal bending and rotating of the supportmember in accordance with the equation:λ=λ_(o)[1+(1−P)r cos(θ)/R] Where: λ_(o)=an unstrained wavelength of theoptical waveguide grating; P=a strain optic coefficient of the opticalwaveguide grating; R=a radius of the bend in the support member; r=adistance of the optical waveguide grating from the support member; andθ=an angle of rotation about the centroidal axis of the support member.2. The apparatus of claim 1, wherein the support member is configuredfor simultaneous longitudinal bending and rotating about the centroidalaxis.
 3. The apparatus of claim 1, wherein the support member is alongitudinal rod.
 4. The apparatus of claim 3, wherein the longitudinalrod has a circular cross-section.
 5. The apparatus of claim 3, wherein alongitudinal axis of the optical waveguide grating is parallel to thecentroidal axis of the rod.
 6. The apparatus of claim 1, wherein anoptical waveguide grating is wound about the centroidal axis of thesupport member.
 7. The apparatus of claim 6, further comprising twooptical waveguide gratings wound about the centroidal axis of thesupport member.
 8. The apparatus of claim 7, wherein the two opticalwaveguide gratings are wound about the centroidal axis of the supportmember such that they are positioned 180 degrees apart from each otheron the support member.
 9. The apparatus of claim 1, wherein the supportmember includes a cavity extending longitudinally therethrough in whichan optical waveguide can be positioned.
 10. The apparatus of claim 9,wherein the longitudinally extending cavity is parallel to thecentroidal axis of the support member.
 11. The apparatus of claim 9,wherein the longitudinally extending cavity intersects the centroidalaxis of the support member.
 12. The apparatus of claim 1, wherein afiber Bragg grating is attached to the support member.
 13. The apparatusof claim 1, wherein a chirped optical waveguide is attached to thesupport member.
 14. The apparatus of claim 1, wherein an opticalwaveguide grating having a length of 100 mm or greater is attached tothe support member.
 15. The apparatus of claim 14, wherein an opticalwaveguide grating having a length of 1 m or greater is attached to thesupport member.
 16. The apparatus of claim 1, wherein an opticalwaveguide grating is attached to the support member along the entirelength of the grating.
 17. An apparatus for adjusting the spectralresponse of an optical grating in an optical waveguide, the apparatuscomprising: a support member to which a longitudinal optical waveguidegrating can be attached, the support member having a centroidal axis andbeing configured for longitudinal bending and rotating about thecentroidal axis, wherein the optical waveguide grating is helicallywound about the centroidal axis of the support member with a helix angleα in accordance with the relationshipIntrinsic Chirp>λ_(o)*(1−P)*sin(α)/R Where: Intrinsic Chirp=an intrinsicchirp of the optical waveguide grating; λ_(o)=an unstrained wavelengthof the optical waveguide grating; P=a strain optic coefficient of theoptical waveguide grating; α=the helix angle; R=a radius of a bend ofthe centroidal axis of the support member.
 18. A method for adjustingthe spectral response of an optical waveguide grating, the methodcomprising: attaching a longitudinal optical waveguide grating to alongitudinal support member having a centroidal axis, wherein thesupport member is configured for longitudinal bending and rotating aboutthe centroidal axis; and longitudinally bending and rotating the supportmember about the centroidal axis and thereby altering the spectralresponse of the optical waveguide grating, wherein longitudinallybending and rotating the support member about the centroidal axiscomprises controlling the longitudinal bending and rotating of thesupport member in accordance with the equation:λ=λ_(o)(1+(1−P)r cos(θ)/R) Where, for each point on the opticalwaveguide grating: λ_(o)=an unstrained wavelength of the opticalwaveguide grating; P=a strain optic coefficient of the optical waveguide grating; R=a radius of the bend of the centroidal axis; r=adistance of the optical waveguide grating from the centroidal axis; andθ=an angular position of the optical waveguide grating about thecentroidal axis.
 19. The method of claim 18, wherein longitudinallybending and rotating the support member about its centroidal axiscomprises simultaneously bending and rotating the support member. 20.The method of claim 18, wherein altering the spectral response of theoptical waveguide grating comprises altering axial strain in thegrating.
 21. The method of claim 18, further comprising aligning alongitudinal axis of the optical waveguide grating parallel to thecentroidal axis of the longitudinal support member.
 22. The method ofclaim 18, further comprising winding the optical waveguide gratingaround the centroidal axis of the support member.
 23. The method ofclaim 22, further comprising winding two optical waveguide gratingsabout the centroidal axis of the support member.
 24. The method of claim23, wherein winding the two optical waveguide gratings about thecentroidal axis of the support member comprises winding the two opticalgratings 180° apart.
 25. The method of claim 18, wherein attaching alongitudinal optical waveguide grating to a longitudinal support membercomprises attaching the grating along the entire length of the grating.26. The method of claim 18, wherein attaching a longitudinal opticalwaveguide grating comprises attaching an optical waveguide gratingselected from the group consisting essentially of: fiber Bragg gratingsand long period gratings.
 27. The method of claim 18, wherein alteringthe spectral response of the optical grating comprises uniformlyshifting the spectral response of a portion of the optical grating. 28.The method of claim 18, wherein altering the spectral response of theoptical grating comprises altering the chromatic dispersion of a portionof the optical grating.
 29. The method of claim 18, wherein altering thespectral response of the optical grating comprises changing thewavelength and the chromatic dispersion of a portion of the opticalgrating.